Especially in the field of medium- or high-voltage transformers, oil is frequently used to insulate the transformer, for the reasons set out below.
Partial discharge is a well-known phenomenon in electrical apparatuses subjected to medium or high voltages.
A partial discharge is an electric discharge limited to a portion of the insulation of an electrical system and does not therefore cause immediate failure of the system but, more generally, causes its gradual degradation. By their very nature, therefore, partial discharges are substantially limited in extent to a defect in the insulating system.
In light of this, the use of a liquid insulator such as oil has the advantage of allowing convective movements within the oil and thanks to certain chemical processes, this type of insulation is at least partly self-restorative, that is to say, it is capable of at least partly compensating the degradation it undergoes during operation of the transformer.
It is known that partial discharges that take place in the oil cause gases to be formed.
Another factor in the evolution of gases is the reaching of very high temperatures by the oil.
For this reason, diagnostic systems based on the assessment of gas concentration in the oil have been in use for some time to assess the insulation condition of oil-insulated transformers.
In this field, the most advanced solutions involve the use of a membrane permeable to the gas, interposed between a container for the oil and a measuring chamber containing only gas. The measuring chamber receives through the membrane a part of the gas present in the oil.
That way, by separating the measuring chamber from the oil it is possible to place a sensor in the measuring chamber to measure the value of gas concentration in the measuring chamber. The sensor is particularly reliable because it is never in contact with the oil.
This type of configuration, however, makes it necessary to estimate the value of gas concentration in the oil as a function of the value measured in the measuring chamber. In effect, it is not the quantity of interest, namely, the gas concentration in the oil, that is measured directly but a quantity indirectly correlated with it, namely, the concentration of the gases inside the measuring chamber.
This estimated value is derived using suitable processing means which implement formulas that reflect the condition of equilibrium between the concentration of gases in the oil and the concentration of gases in the measuring chamber.
It should be observed, however, that these formulas do not take into account the dynamics of the phenomenon by which the gases pass through the membrane from the oil to the measuring chamber.
The aforesaid technical solutions therefore have certain shortcomings.
First of all, there is the risk that the values estimated for gas concentration in the oil will differ considerably from the real values. Typically, the risk is such that the values will be underestimated, leading to serious diagnostic assessment errors when the estimated values are interpreted.
Moreover, there is also the risk that such an estimation of the values of gas concentration in the oil will not allow particularly intense partial discharge phenomena to be identified at all or to be identified with an unacceptable delay. That makes such measuring systems somewhat unreliable for diagnostic purposes.
Lastly, the aforesaid prior art systems are not very precise or reliable during the steps of checking and adjusting (setting up) the devices themselves, where particularly rapid transients in the variation of gas concentration in the oil are created.
In effect, it should be observed that some prior art systems contemplate a calibration procedure to take into account the dynamics of the phenomenon by which gas passes through the membrane from the oil to the measuring chamber.
These calibration procedures involve setting a first predetermined value of gas concentration in the oil and measuring the corresponding value of gas concentration in the measuring chamber, then setting a second predetermined value of gas concentration in the oil (greater than the first value) and measuring the corresponding value of gas concentration in the measuring chamber, and so on.
That provides a plurality of experimental values of gas concentration in the to measuring chamber, each corresponding to a known value of gas concentration in the oil.
These experimental points are then interpolated to derive a coefficient of proportionality, that is to say, a coefficient of calibration.
These calibration procedures, however, are also not free of disadvantages because they are highly time-consuming and, in any case, involve a somewhat heavy approximation, with the result that estimation of the value of gas concentration in the oil is relatively imprecise.
It should also be observed that systems which involve measuring gas concentration in a measuring chamber separated from the oil by a membrane have a further drawback due to possible gas saturation in the measuring chamber.
In effect, if the concentration of the gas to be measured in the measuring chamber reaches saturation, the values measured by the sensor are not reliable.
To avoid saturation, it is necessary to discharge at least part of the gases present in the measuring chamber. This, however, leads to transients related to the transfer of the gases through the membrane, thus further increasing the risks of error in estimating the values of gas concentration in the oil, as described above.